In recent years, laser light has been used in various applications; for example, laser light has been used in the cutting and working of metals, and as a light source in photolithographic devices used in semiconductor manufacturing apparatuses. Furthermore, laser light has been used in various types of measuring instruments, and in operations and treatment devices used in surgery, ophthalmology, dentistry, and the like.
However, in the case of ArF excimer laser oscillators, the oscillators are constructed with argon gas, fluorine gas, neon gas, and the like sealed inside the chamber, and these gases must be tightly sealed. Furthermore, since these respective gases must also be loaded into the apparatus and recovered, there is a problem in that the apparatus tends to become large in size and complicated. Furthermore, in order to maintain a specified laser light generating performance in an ArF excimer laser oscillator, the internal gases must be periodically replaced, or the apparatus must be periodically overhauled.
Accordingly, it is desirable to use a solid-state laser as a laser light source instead of such an excimer laser. However, the wavelength of the laser light that is emitted from a solid-state laser ranges from the visible region to the infrared region; therefore, this wavelength is too long to be suitable for use, for example, in an inspection device. Accordingly, a method has been developed in which long-wavelength light emitted from such a solid-state laser is used after being converted into short-wavelength ultraviolet light (e.g., an eighth harmonic wave) using a non-linear optical crystal. For example, such a method is described in Japanese Patent Application Laid-Open No. 2001-353176 (Patent Reference 1).
An outline of the optical system of such a laser apparatus is shown in FIG. 9. In this figure, the objects indicated by oval shapes are collimator lenses and focusing lenses; a description of these lenses is omitted. Furthermore, P polarization is indicated by an arrow symbol, S polarization is indicated by symbols showing a dot inside a circle, the fundamental wave is indicated by ω, and the n-th harmonic wave is indicated by nω.
In this example, fundamental light (wavelength: 1547 nm) emitted from a DFB laser (not shown in the figure) is amplified by an erbium doped fiber amplifier (EDFA) 51 and converted into P polarization, and is then incident on a second harmonic wave forming optical element (PPLN crystal) 52. A second harmonic wave of P polarization is generated and output from the second harmonic wave forming optical element 52 along with the fundamental wave.
This fundamental wave and second harmonic wave are incident on a third harmonic wave forming optical element (LBO crystal) 53. A third harmonic wave of S polarization is generated and output from the third harmonic wave forming optical element 53 along with the fundamental and second harmonic waves . This light passes through a dichroic mirror 54 and separate into the fundamental wave and the second/third harmonic waves. The separated second harmonic wave/third harmonic wave passes through a 2-wavelength wave plate 55; in this case, the second harmonic wave is converted into S polarization. Both the second harmonic wave converted into S polarization and the third harmonic wave are incident on a fifth harmonic wave forming optical element (LBO crystal) 56. A fifth harmonic wave of P polarization is generated and output from the fifth harmonic wave forming optical element 56 along with the second harmonic and third harmonic waves.
The second harmonic wave, third harmonic wave and fifth harmonic wave pass through a dichroic mirror 57, so that the second harmonic wave and fifth harmonic wave are separated. The separated fifth harmonic wave is reflected by a mirror 58, and is subjected to beam shaping by cylindrical lenses 59 and 60. Generally, because of walk-off, the fifth harmonic wave generated in the fifth harmonic wave forming optical element 56 has an elliptical cross-sectional shape, so that the focusing characteristics are poor “as is,” and the wave cannot be used in the next wavelength conversion. Accordingly, this elliptical cross-sectional shape is shaped into a circular shape by the cylindrical lenses 59 and 60.
The second harmonic wave separated by the dichroic mirror 57 is converted into P polarization by passing through a ½-wave plate 61, and is reflected by a mirror 62. This light is then placed on the same optical path as the fifth harmonic wave by a dichroic mirror 63. The dichroic mirror 63 allows the second harmonic wave to pass through and reflects the fifth harmonic wave. The second and fifth harmonic waves are incident on a seventh harmonic wave forming optical element (CLBO crystal) 64. A seventh harmonic wave of S polarization is generated and output from the seventh harmonic wave forming optical element 64 along with the second and fifth harmonic waves. Because of walk-off, this seventh harmonic wave also has an elliptical cross-sectional shape, and therefore has poor focusing characteristics “as is,” so that this wave cannot be used in the next wavelength conversion. Accordingly, this elliptical cross-sectional shape is shaped into a circular shape by means of cylindrical lenses 65 and 66.
Meanwhile, the fundamental wave separated by the dichroic mirror 54 is reflected by a mirror 67, and is converted into S polarization by passing through a ½-wave plate 68. This light is then placed on the same optical path as the seventh harmonic wave by a dichroic mirror 69. The dichroic mirror 69 allows the fundamental wave to pass through and reflects the seventh harmonic wave. The fundamental and seventh harmonic waves are incident on an eighth harmonic wave forming optical element (CLBO crystal) 70. An eighth harmonic wave of P polarization is generated and output from the eighth harmonic wave forming optical element 70 along with the fundamental and seventh harmonic waves.
However, in the optical system shown in FIG. 9, the following problems arise: namely, the optical elements that are used are numerous and complicated; furthermore, the dichroic mirror 69 used to combine the fundamental wave and seventh harmonic wave is required. When the wavelength of the eighth harmonic wave is 193 nm, the wavelength of the seventh harmonic wave is 221 nm. For such deep ultraviolet light, dichroic mirrors generally show problems in terms of durability. Moreover, an adjustment is needed to superimpose the fundamental and seventh harmonic waves by means of the dichroic mirror 69, which is difficult.